Try to do the following:Press the power button until you can go to the power off menu -> At this menu hold the Power Off button until the option for the save mode boot appears. -> Enter save mode and see if this helps (It gives the system a kick behind the scenes ).

After that simply restart. If that doesn't work I recommend to manually search for updates (Settings -> Search for update ).

Also, did you install an app that might have triggered the slowness issues? If yes, uninstall the app in case.